MDEV-7317: Make an index ignorable to the optimizer

This feature adds the functionality of ignorability for indexes.
Indexes are not ignored be default.

To control index ignorability explicitly for a new index,
use IGNORE or NOT IGNORE as part of the index definition for
CREATE TABLE, CREATE INDEX, or ALTER TABLE.

Primary keys (explicit or implicit) cannot be made ignorable.

The table INFORMATION_SCHEMA.STATISTICS get a new column named IGNORED that
would store whether an index needs to be ignored or not.

sql/sql_class.h

@@ -381,6 +381,29 @@ public:
 };
 
 
+/* An ALTER INDEX operation that changes the ignorability of an index. */
+class Alter_index_ignorability: public Sql_alloc
+{
+public:
+  Alter_index_ignorability(const char *name, bool is_ignored) :
+    m_name(name), m_is_ignored(is_ignored)
+  {
+    assert(name != NULL);
+  }
+
+  const char *name() const { return m_name; }
+
+  /* The ignorability after the operation is performed. */
+  bool is_ignored() const { return m_is_ignored; }
+  Alter_index_ignorability *clone(MEM_ROOT *mem_root) const
+    { return new (mem_root) Alter_index_ignorability(*this); }
+
+private:
+  const char *m_name;
+  bool m_is_ignored;
+};
+
+
 class Key :public Sql_alloc, public DDL_options {
 public:
   enum Keytype { PRIMARY, UNIQUE, MULTIPLE, FULLTEXT, SPATIAL, FOREIGN_KEY};